Nonwoven structures have long been known and used, for example, in papermaking and felting operations. More recently, alternative techniques have been used to form coherent webs of fibrous materials. For example, nonwoven structures can be made using cotton processing technology, including the use of cards and garnets. Carded webs tend to be light weight. To make thicker webs, multiple cards, transverse folding of the web or xe2x80x9ccrosslappingxe2x80x9d can be used. Garnets can also be used to make a thick web from one or more fibers and/or fabric waste.
Airlaid webs represent still another approach to making nonwoven products. There, a heavy pulp sheet is defibered in a hammermill or pin mill into individual pulp fibers in an air stream. The air borne dispersed fibers are condensed, via vacuum, onto a porous belt, forming a planar web. The fibers are deposited, in a horizontal orientation, on the porous belt. Multiple layers can be built up, but there is little strength between layers.
The various products made using these techniques, because of their limited strength, are often further treated by a variety of bonding techniques. Mechanical bonding techniques have included needle punching, stitch bonding, and hydroentangling. Chemical bonding techniques generally involve a latex application. In thermal bonding techniques, a fusible substance, generally a powder or fiber, is used to form a support of unbonded fibers into a connected network.
While certain of the mechanical processing techniques described above, such as needle felting, stitchbonding and hydroentangling, can provide some strength in the thickness direction, they do not function on an individual fiber basis, and crosslapped structures, or even those which have been treated with latex bonding or using binder fiber, still have little strength in the thickness direction, that is, the direction normal or perpendicular to the major plane of the web. Such products accordingly have limited utility in multiple use applications.
The present invention provides a process for producing nonwoven products that have significant strength in the direction normal or perpendicular to their planes, but without the time consuming steps of previously used techniques such as needle punching, stitch bonding and the like.
Specifically, the instant invention provides a process for forming a web of fibrous material comprising:
(a) admixing about from 50 to 95% by weight of at least one support fiber for a period sufficient to disentangle and open the fibers and simultaneously or subsequently admixing therewith about from 5 to 50% by weight of at least one binder fiber to provide a substantially homogeneous mixture of fibers;
(b) conveying the mixture of fibers into a shaker chute positioned at an angle of about from 90 to 150 degrees with respect to horizontal;
(c) oscillating the fibers in the shaker chute for a period of about from 5 seconds to 1 minute;
(d) depositing the fiber web onto a substantially horizontal planar surface at a substantially uniform thickness of about from xe2x85x9 to 6 inches;
(e) heating the fibers for a time and at a temperature sufficient to fuse at least some of the fibers; and
(f) cooling the resulting web to a substantially ambient temperature.
The present invention also provides an apparatus for making the fibrous webs and the resulting bonded webs having machine direction and transverse direction axes forming a major plane, and a substantially homogeneous upper surface and comprising fused fibers, in which the Tensile Strength of the web in the direction normal to the major plane is about from 25% to 120% of the Tensile Strength in the machine direction in the major plane of the web.